1. Field of the Invention
The present invention relates to a process for treatment of a fluid comprising at least one chemical contaminant.
2. Description of the Prior Art
There are a number of known techniques available for purifying large volumes of fluids (i.e. liquids or gases); however limitations often are associated with these techniques.
For example, volatile organic compounds in liquids (e.g. ground water) can be removed by air stripping. This process is limited to removal of only those chemicals which can be partitioned between the liquid and gas phases. Moreover, such a process converts a liquid phase contamination problem to a gas phase contamination problem. In general, gas phase contamination problems are more difficult to deal with as they are unseen and can present a relatively severe environmental risk. The off-gases from air stripping processes must themselves be treated in most cases.
Another technique for purifying large volumes of fluids is adsorption of contaminating chemicals using, for example, solid phase granular activated carbon. Such techniques can become deficient when there is a need to replace the solid adsorbent material or, at the very least, regenerate it at frequent intervals thereby increasing the overall cost of the technique. Moreover, when such adsorbents are regenerated, cost intensive thermal destruction techniques are typically required and can potentially create a new contamination problem by release of undesirable pyrolysis products in the flue gases. If solvent extraction techniques are utilized for regeneration of the adsorbents, care must be exercised to ensure that residual solvent is removed from the adsorbent material prior to use. Regeneration of granular activated carbon by wet oxidation or extraction with supercritical fluids (e.g. liquified carbon dioxide) is costly and generally not suitable for on-site regeneration.
Yet another technique for purification of fluids involves addition of oxidants directly to the primary fluid. This technique is unable to efficiently remove contaminants which are present in low concentrations. This is due to the fact that the rates of reaction with oxidants are diffusion controlled. Thus, acceptable rates of reaction may typically require increased concentration of oxidants which results in an overall increase of the cost of the technique, is potentially hazardous and creates a need to dispose of the excess oxidant. Another problem associated with this technique is the relatively high probability that the reactive oxidant species will react with a quenching species prior to reaction with the target contaminant molecule. Again, this necessitates increasing the concentration of oxidant and bearing the additional expense associated therewith. Yet another problem associated with this technique is the difficulty associated with oxidation by-products and end-products formed during the treatment. Specifically, these species can be altered with changes in the contaminant/oxidant ration or with changes in process conditions. Since these species are created in the bulk fluid being treated, it is necessary to monitor the fluid volume being treated for control of formation of these species. In the case of complex mixtures of organic compounds in the primary fluid, the mixture of products can become even more complex, and monitoring these products can become exceedingly difficult. Further, if the concentration of these species in the fluid becomes too high, additional processing of the bulk fluid is required with the concurrent expense thereof.
Another known technique of removing contaminants from fluids comprises the use of illuminated photocatalysts such as titanium dioxide. One of the inventors named herein first published fifteen years ago on the ability of ultraviolet illuminated titanium dioxide to destroy organic contaminants in water [Carey et al., Dull. Environ. Contam. Toxicol. 16, 697, (1976) ]. Since that time, many publications, including a number of patents, have described how titanium dioxide may be used in water purification. However, heretofore, none of the prior processes known to the applicant has emerged as a commercially viable process. It is believed that there exist at least five reasons why prior processes have not been commercially successful.
First, current evidence supports the notion that destruction of organic contaminants occurs on the surface of the photocatalyst, and therefore an increase in surface area is required for high rates of reaction. To achieve this, slurries of colloidal titanium dioxide have typically been used in many processes; however, the recovery of the colloidal photocatalyst in the discharged effluent has not been cost efficient for high volume applications.
Second, immobilization of titanium dioxide on a support within the photoreactor has been suggested and has resolved the retention problem described in the previous paragraph. However, this solution has come at the expense of increasing mass transfer problems associated with movement of the contaminants in water to the immobilizing support which was more distantly spaced (for light penetration purposes) than the dispersed colloidal particles. Immobilization also creates a lack of uniformity in irradiation of photocatalytic particles which are immobilized at different distances and with different orientations to the light source. Larger photoreactors would therefore be needed to cope with the inefficiencies introduced by immobilization. With colloidal slurries (i.e. previous paragraph), mixing provided all particles with equal probability of being in low and high light intensity regions of the photoreactor.
Third, when treating dilute solutions, the resulting mass transfer problems reduce significantly the rate of chemical destruction in both the slurried and immobilized titanium dioxide processes described above. Unfortunately, these problems serve to restrict the fluid volumes treatable by a given amount of photocatalyst in any given time, and necessitate the use of large reactors to handle large fluid volumes. Since many applications, such as municipal drinking water purification, require treatment of large volumes with low concentrations of contaminants, prior art processes involving the use of photocatalysts in this manner are severely limited for such applications.
Fourth, the large surface area on closely spaced particles and other control parameters which favour purifying the fluid by loading of dilute contaminants onto the surface of the photocatalyst (a rate influencing step in the overall photocatalytic process) are not, except by coincidence, generally compatible with process control parameters such as ready light penetration into the fluid for the light-mediated step in the overall photocatalytic process.
Fifth, as in the case of using oxidants such as hydrogen peroxide or ozone directly on the fluid of interest, there is the risk that, in the absence of adequate process control when using photocatalysts, undesirable by-products or end-products will be released in the treated effluent. This is true for both the slurried and immobilized processes.
In light of the foregoing, it would be desirable to have a process capable of purifying fluids while minimizing or eliminating the above-mentioned deficiencies of the prior art. Ideally, such a process would be useful to remove chemical contaminants from fluids in a relatively simple and efficient manner, and would decompose or transform the removed contaminants to dischargeable and innocuous or otherwise desirable products which could, at the discretion of the user, be diluted with purified fluid or captured for further processing such as microbial treatment. Further, it would be advantageous if such a process could be easily adapted for purifying liquid and/or gas phase fluids.